1. Field of Invention
The present invention relates to a readout circuit more particularly to a signal readout circuit for an amperometric sensor.
2. Related Art
A blood glucose meter is used as an index indicating daily health status for ordinary people, and also used as a device for evaluating the prevention and curing effects for people in high risk groups or already being sick. Recently, the most common inspection method is conducted by an electrochemical-based amperometric sensor.
The amperometric sensor is generally divided into dual-electrode sensor and tri-electrode sensor. FIG. 1 is a readout circuit for a common dual-electrode amperometric sensor, which uses the virtual short circuit characteristic of OP2 to pass the voltage of the working electrode to the ground, and uses OP1 as a voltage follower to provide the voltage input.
Moreover, Turner, R. F. B.; Harrison, D. J.; Baltes, H. P et al. also disclosed a readout circuit for a dual-electrode amperometric sensor in “A CMOS potentiostat for amperometric chemical sensors”; IEEE Journal of Solid-State Circuits; Volume 22, Issue 3, June 1987 Page(s): 473-478. In addition, Breten, M.; Lehmann, T.; Braun, E. et al. disclosed a readout circuit for a dual-electrode amperometric sensor in “Integrating data converters for picoampere currents from electrochemical transducers”; ISCAS 2000 IEEE International Symposium on Circuit and system; Volume 5, 28-31 May 2000 Page(s): 709-712.
Narula, H. S.; Harris, J. G. et al. published similar circuits in “Integrated VLSI potentiostat for cyclic voltammetry in electrolytic reactions”; IEEE Computer society Annual Symposium on VLSI; 19-20 Feb. 2004 Page(s): 268-270.
The dual-electrode sensing architecture has some problems, and the main disadvantage lies in that when the solution is in reaction, anion moves towards anode and cation moves towards cathode, such that the charged ions are attached to the surface of the reference electrode, so as to produce a concentration polarization phenomenon which further affects the electric potential changes in the solution and is more obvious when the measured current is larger than 10 uA.
FIG. 2 shows another constant potential instrument used in a readout circuit for a tri-electrode amperometric sensor. The circuit is characterized in an amplifier OP1 which causes that the working electrode must be grounded. Thus, when the voltage across the electrode is to be detected, it is only necessary to measure a voltage on the reference electrode, and perform a synergism on the measured voltage and an input voltage (Vin). Then, the result is amplified by an amplifier OP2, and fed back to control a counter electrode (CE). The current signal of the reaction current is converted into a voltage signal after passing through a resistor Rm.
In fact, the voltage on working electrode (WE) is not virtually grounded, when the frequency increases, the voltage on the WE will increase accordingly. This characteristic is similar to inductance, and the inductance is easy to produce resonance with the double-layer capacitor between electrodes, thus causing the whole circuit to be unstable.
In view of the above various limitations and changes of era, the requirements on the effectiveness of implementing medical treatments on medical spots are rather urgent. Under such circumstance, the detection system having small equipments, convenient operations, portability, and real time capability gradually becomes a trend, so the electrochemical analysis method with developing potential of meeting the above requirements is highly favored recently.